1. Technical Field
The disclosed embodiments generally pertain to sheet registration systems and methods for operating such systems. Specifically, the disclosed embodiments pertain to methods and systems for registering sheets using a gain-scheduled feedback control scheme based on the pseudo-linearized system.
2. Background
Sheet registration systems are presently employed to align sheets in a device. For example, high-speed printing devices typically include a sheet registration system to align paper sheets as they are transported from the storage tray to the printing area.
Sheet registration systems typically use sensors to detect a location of a sheet at various points during its transport. Sensors are often used to detect a leading edge of the sheet and/or a side of the sheet to determine the orientation of the sheet as it passes over the sensors. Based on the information retrieved from the sensors, the angular velocity of one or more nips can be modified to correct the alignment of the sheet.
A nip is formed by the squeezing together of two rolls, typically an idler roll and drive roll, thereby creating a rotating device used to propel a sheet in a process direction by its passing between the rolls. An active nip is a nip rotated by a motor that can cause the nip to rotate at a variable nip velocity. Typically, a sheet registration system includes at least two active nips having separate motors. As such, by altering the angular velocities at which the two active nips are rotated, the sheet registration system may register (orient) a sheet that is sensed by the sensors to be misaligned.
Numerous sheet registration systems have been developed. For example, the sheet registration system described in U.S. Pat. No. 4,971,304 to Lofthus, which is incorporated herein by reference in its entirety, describes a system incorporating an array of sensors and two active nips. The active sheet registration system provides deskewing and registration of sheets along a process path having an X, Y and θ coordinate system. Sheet drivers are independently controllable to selectively provide differential and non-differential driving of the sheet in accordance with the position of the sheet as sensed by the array of sensors. The sheet is driven non-differentially until the initial random skew is measured. The sheet is then driven differentially to correct the measured skew and to induce a known skew. The sheet is then driven non-differentially until a side edge is detected, whereupon the sheet is driven differentially to compensate for the known skew. Upon final deskewing, the sheet is driven non-differentially outwardly from the deskewing and registration arrangement.
A second sheet registration system is described in U.S. Pat. No. 5,678,159 to Williams et al., which is incorporated herein by reference in its entirety. U.S. Pat. No. 5,678,159 describes a deskewing and registering device for an electrophotographic printing machine. A single set of sensors determines the position and skew of a sheet in a paper process path and generates signals indicative thereof. A pair of independently driven nips forwards the sheet to a registration position in skew and at the proper time based on signals from a controller which interprets the position signals and generates the motor control signals. An additional set of sensors can be used at the registration position to provide feedback for updating the control signals as rolls wear or different substrates having different coefficients of friction are used.
In addition, U.S. Pat. No. 5,887,996 to Castelli et al., which is incorporated herein by reference in its entirety, describes an electrophotographic printing machine having a device for registering and deskewing a sheet along a paper process path including a single sensor located along an edge of the paper process path. The sensor is used to sense a position of a sheet in the paper path and to generate a signal indicative thereof. A pair of independently driven nips is located in the paper path for forwarding a sheet therealong. A controller receives signals from the sensor and generates motor control drive signals for the pair of independently driven nips. The drive signals are used to deskew and register a sheet at a registration position in the paper path.
FIGS. 1A and 1B depict an exemplary sheet registration device according to the known art. The sheet registration device 100 includes two nips 105, 110 which are independently driven by corresponding motors 115, 120. The resulting 2-actuator device embodies a simple registration device that enables sheet registration having three degrees of freedom. The under-actuated (i.e., fewer actuators than degrees of freedom) nature makes the registration device 100 a nonholonomic and nonlinear system that cannot be controlled directly with conventional linear techniques. The control for such a system, and indeed for each of the above described systems, employs open-loop (feed-forward) motion planning.
FIG. 2 depicts an exemplary open-loop motion planning control process according to the known art. One or more sensors, such as PE2, CCD1 and CCD2 shown in FIG. 1B, are used to determine the input (initial) sheet position 125 when the lead edge of the sheet is first detected by PE2 (as represented in FIG. 1B). Note the sheet position, as described, includes the process (the direction that the sheet is intended to be directed), lateral (cross-process), and skew (orientation) degrees of freedom for the sheet. An open-loop motion planner 205 interprets the information retrieved from the sensors as the input position and calculates a set of desired velocity profiles ωd that will steer the sheet along a viable path to the final registered position if perfectly tracked (i.e., assuming that no slippage or other errors occur). One or more motor controllers 210 are used to control the desired velocities ωd. The one or more motor controllers 210 generate motor control signals um for the motors 115, 120. The motor control signals um determine the angular velocities ω at which each corresponding nip 105, 110 is rotated. For example, a pulse width modulated voltage can be created for a DC brushless servo motor based on um1 to track a desired velocity ω1. Alternately, any of a stepper motor, an AC servo motor, a DC brush servo motor, and other motors known to those of ordinary skill in the art can be used. The sheet velocity at each nip 105, 110 is computed as the radius (c) of the drive roll multiplied by the angular velocity of the roll (ω1 for 105 and ω2 for 110). By matching the angular velocities of the nips 105, 110 to ωd, sheet registration can be achieved.
Although the sheet is not monitored for path conformance during the process, an additional set of sensors, such as PEL, CCDL and CCD1 in FIG. 1B, can be placed at the end of the registration system 100 to provide a snapshot of the output (final) sheet position to update the motion planning algorithm based on a learning algorithm. However, because path conformance is not monitored, error conditions that occur in an open-loop system may result in errors in the output sheet position that require multiple sheets to correct. In addition, although learning can be used to remove repetitive and slow-changing sources of error, the open-loop nature of the underlying motion planning remains vulnerable to non-repetitive and fast-changing sources of error. Accordingly, the sheet registration system may improperly register the sheet due to slippage or other errors in the system.
Systems and methods for improving the registration of misaligned sheets in a sheet registration system, for using feedback control of a pseudo-linearized system in a sheet registration system, and/or for scheduling gain in a sheet registration system to control the resulting nip forces and sheet tail wag within design constraints while converging the sheet to a desired trajectory within a pre-determined time would be desirable.